Please correct me if I am wrong, but the only mention of other scientists who came to the same conclusion that I can find in my copy is Darwin's comments on Wallace in the introduction. If you can point me to where you found the others, I can update my answer accordingly.
The reason we remember Darwin over those who preceded him comes down to a matter of audience. Bynum, in his introduction to the 2009 Penguin Classics edition of On the Origin of Species notes that 'Darwin wrote for a general audience.' ^1 Taken as such, the book was an immense success, its first printing sold out on the first day of sale, and it has never gone out of print since. Darwin's work gave his case in understandable language, easy enough that a non-expert could understand. Darwin's friend, fellow scientist and populariser of science, Thomas Huxley, wrote after reading the book: ‘How extremely stupid not to have thought of that!' ^2 Darwin's writing easily captured the Victorian popular imagination. In addition to being easy to read, ‘On the Origin of Species’ also worked very well with the then-popular Malthusian idea of population growth: the belief that population will continue to grow faster than resources can handle unless checked or stopped. The term ‘Darwinism’ was coined and popularised during Darwin’s time and reflects the manner in which he caught the attention of a wide audience.
Wallace, another friend of Darwin's, had sent Darwin a similar paper to be sent to the Linnaean Society of London. Darwin did as any good friend would, and sent it to be published. The contact to whom Darwin sent the paper as instructed by Wallace, Sir Charles Lyall, was familiar with the work of both men. A presentation on the theory was then organised, at which both men would present their findings and take credit for the conclusion each had come to separately. ^3 It was this event that triggered Darwin to write ‘On the Origin of Species’. ^4
Wallace himself wrote for a more scientific audience. He did write a number of books, but most were on niche topics, or observations from small areas. Wallace’s lack of popular appear allowed for him to be easily eclipsed by Darwin. This was not helped by the exclusive nature of the Linnaean Society, who restricted membership to those voted in by the pre-existing membership. ^5
Notes
Bynum, W., 'Introduction' in On the Origin of Species, (London: Penguin Books, 2009) p.xv
Quoted in Bynum, p.xvi
Darwin, C., On the Origins of Species, (London: Penguin, [1859] 2009) p.1
As above pp.1-3
Belknap, G., 'Illustrating natural history: images, periodicals, and the making of nineteenth-century scientific communities', The British Journal of the History of Science, Vol. 51, 3, 2018, pp.395-422
I was not totally sure what you were referring to in Darwin's work, but I assume it is the "Historical Sketch" that began to be included in third edition (1861 American) (and not the first edition, which is kind of interesting).
There are two questions here: 1. Why did Darwin append this "Historical Sketch," which discusses precursor ideas (Lamarck, Saint Hilaire, Vestiges, Spencer, etc.) to his work? and 2. Why do we talk of Darwin more than any of the others when we think of the history of evolution?
I do not know the answer to #1 — none of the main Darwin biographies (e.g. Desmond and Moore, or Browne) that I have access to at the moment say anything more than that he added it. One could speculate: giving historical context to his already-famous book (at that point) both looks generous and also clearly works to differentiate his ideas from the precursors (Darwin was particularly annoyed that many people did not understand why he and Lamarck had radically different views, or why his book was something quite different from the pop-sci of Vestiges of the Natural History of Creation). But it also could have just been something his publisher asked for. I don't know. Darwin made many changes between different versions of Origin and it is not always clear what motivated him.
The second question is a little easier to answer. Darwin's work was certainly built upon the work of others — like all science. Darwin himself would never have denied this. But Darwin's work was also quite different from most of them.
Darwin's work was scientific but semi-popular. The two "competitors" he had for the English public imagination on evolutionary ideas were Lamarck and an anonymously written book called Vestiges of the Natural History of Creation. Lamarck's theories were well-known but also known to be scientifically flawed. Lamarck's work is still used as a "foil" to Darwin's in biology textbooks today. Lamarck is usually given credit for having made the first strong scientific claims to the idea that species had evolved over time, but his mechanism for this evolution was not Darwin's: Lamarck believed that environmental changes could be inherited. So the child of two body-builders, in Lamarck's view, would likely already be born with a body-builder's body (say that three times fast). Or that giraffes got their long necks by subsequent generations stretching them. There is more to Lamarck's theory than this (it is popularly misunderstood and only used in this kind of crude way to contrast with Darwin), but this is the gist of the issue.
The examination of mummified animals from Egyptian tombs had convinced other scientists that species change had not occurred within the past several thousand years, and Lamarck's work had become politically associated with the French Revolution and its excesses. The result is that while Lamarck was well-known, and perhaps a bit daring, it was generally thought of as wrong. (It is an interesting thing to ask whether Darwin thought Lamarck's mechanism of inherited environmental trait was wrong — even Darwin thought it was likely that some environmental activity was inherited. We tend not to emphasize this in modern biology textbooks, however!)
Vestiges was a popular book that traced a "cosmic evolution" from nebula and stars all the way through people. It was light on science and high on ambition. There were, for all of Darwin's life, more copies of Vestiges in England than there were copies of Origin, as a point of reference. Again it was considered politically daring but scientifically incorrect, but it paved the way for the British public in particular to start thinking about evolutionary theories.
There were more precursors that Darwin himself could have named, but they would not have been well-known by any other than scientists. Darwin himself took pains to emphasize throughout Origin, not just the "Historical Sketch," that his ideas were not just picked out of nothing. Why do this? Because he was trying to convince people that he was approaching this seriously (unlike the author of Vestiges) and that he had the evidence on his side (unlike Lamarck). Which is a very fine line from a "credit" perspective, one Darwin tried to be very careful about: he wanted to acknowledge what was continuous with other thinkers, but also emphasize where he had taken a radical detour.
So, anyway, why do we remember Darwin primarily? Because these previous thinkers failed a) to convince the broader scientific community of the fact of evolution, and b) because Darwin's mechanism for evolution (natural selection) became the basis of what we use today. So Darwin today gets credit for both of these things: making scientists take the idea of evolution seriously, and the mechanism of natural selection.
But it is an interesting historical footnote that this vision of him is not one that solidified until the 1930s. Prior to that, Darwin was only given credit for the first of these (making people take evolution seriously). Why? Because after Darwin's death, the scientific community largely decided that natural selection did not account for evolution. It was viewed as too slow, and too incompatible with ideas about heredity, to be the "answer." So Darwin was still given lots of credit, but often with a note about how he couldn't have been totally correct. Historians call this period "the eclipse of Darwinism."
It was only with the creation of the modern evolutionary synthesis, which merged Darwinian natural selection with the field of population genetics, that Darwin once again came out on top, and became the doubly-revered figure he is today. It is an interesting question to ask why almost nobody knows the names of the creators of the modern synthesis, by comparison (Dobhzansky, Mayer, Huxley, among others). It may be because there were many of them, and that the modern synthesis is not a single "idea" but a combination of techniques and insights that emerged over time. It may be related to the way in which we like to codify individual scientific "geniuses." Or there may be some other reason that historical memory got re-cast — as an example, the reason everyone knows about Gregor Mendel is not that Mendel's papers were themselves so important, but that the "rediscoverers" of Mendel's work were petty about credit and would prefer it went to someone long dead than one of their living scientific colleagues (seriously!). Which is only to say: the reasons why scientists, much less popular writers, give one person "credit" for things can be complicated.
Historians of science know well how complicated credit is, and that virtually all scientific ideas have deep precursors and complex webs of possible attribution, which typically gets boiled down to singular geniuses both inside and outside the literature of science. Pointing to historical "luminaries" does work both for scientists (it is part of the training of scientists, to give a model to emulate but also to create a "hero" of sorts) and for the public (it builds up a mystique of scientific genius that makes the work seem credible and unassailable). It is worth noting that within Darwin's lifetime, his allies used him in this way as well: Darwin himself was already scientifically respected (for his pre-evolution work), and could be "mobilized" in their own battles against other scientists, religious orthodoxy, and in favor of the new emerging "profession" of science (T.H. Huxley in particular approached it in this way). None of his precursors could be mobilized in this fashion, at least not in England. (In France, they continued to use Lamarck as their "hero" for many decades.)
None of which is to downplay Darwin's actual contributions. He wasn't just some random bump along the road in evolutionary thinking. He brought together a huge amount of evidence, previous thought, and original thought, to create not just one but several major contributions that set the stage for new questions about biological evolution, human origins, human diversity, heredity, and the age of the Earth. And he managed to do it in a way that captivated not only the scientific community, but lay populations as well. That all gets boiled down into "convinced people evolution was an important thing, and also natural selection was the mechanism for it," but he made huge contributions. If you're going to look for a "hero" of evolution, there is a legitimate argument that he outshines all of the "competitors" significantly, and the fact that later work validated many of his arguments (though not all of them) certainly contributes to that. My point is just that credit is a trickier thing than, "this guy did the most important work!" It's a way of making certain kinds of historical arguments, and a way of distilling the work of science down into a single name, and that serves various ends.
Anyway — the above is a little jumbled, but hopefully it points a bit of the way towards thinking about why credit is popularly and even scientifically assigned in this way. Some resources: on Darwin, the best biographies are Janet Browne's (Voyaging and The Power of Place) and Desmond & Moore (Darwin). On Vestiges and the context of evolutionary thought in England pre-Darwin, see Secord (Victorian Sensation). On the development of evolutionary theory, esp. the "Eclipse of Darwinism" and modern synthesis, see Bowler, Evolution: The History of an Idea. For thoughts on scientific credit in general, see Biagioli and Galison, Scientific Authorship.
All comments have been very helpful. I admit my question comes from ignorance of these previous scientists, their ideas and how their work was seen by the scientific community and the public. Now I understand how easily Darwin’s work could be read by the lay audience and how the method (natural selection) was different than acquired traits (bodybuilders muscles, etc.) Thank you all.